Technical Field
The present invention relates to an image reading device, such as an image scanner, and an image reading lens adapted to read an original image as a part of an image forming apparatus, such as a facsimile and a digital copying machine. Further, the present invention relates to an image forming apparatus including the image reading device.
Description of Related Art
Image reading devices, which are installed into image scanners, facsimiles, digital copying machines or the like to read original images, are configured to form reduced optical images using image reading lenses and to photograph the reduced optical images by solid-state image pickup elements (e.g., CCD image sensors) so as to make the images into electronic image data. Further, to read the original images as color images, the solid-state image pickup elements are configured with so-called three-line CCD image sensors having light-receiving elements arranged in three lines. Each of the light-receiving elements includes, for example, a red filter, a green filter, and a blue filter. By forming the reduced optical images of the original documents onto the light-receiving surfaces of the solid-state image pickup elements, the image reading devices perform color separation of the images into three primary colors and convert the color image data into signals. In general, the image reading lenses are required to have a high contrast in a high special frequency region on the image plane and a high aperture efficiency up to the periphery part of a viewing angle. Additionally, in order to appropriately read color original documents, the image-forming positions in the optical direction of the primary colors, i.e., red, green and yellow, on the light-receiving surfaces need to be identical to each other. That is, the chromatic aberration of each primary color needs to be corrected accurately.
On the other hand, in order to downsize the image reading devices and to accelerate image reading speed thereof, heating value in the vicinity of the photographing part such as the CCD image sensors increases. Further, in order to increase illuminance of light sources to illuminate the original images, the heating value of the light source also increases. As a result, the temperature inside the image reading device increases, and thus the performance of the devices may be deteriorated. Patent Literature 1 (JP2014-174415 A) discloses an image reading lens to restrain the deterioration of the performance caused by temperature changes.